Electronically commutated motor having an outside rotor and an inside position detector

ABSTRACT

An electronically commutated motor having a multiple pole permanent magnet rotor. A position detecting means is located inside of the stator. This arrangement makes it easy to fabricate the stator and the position detecting means from the same sheet of laminated magnetic material with little material loss. A position detecting rotor having a plurality of teeth with shortcircuited conductors thereon modulates the coupling between the primary and secondary windings in the position detecting windings during its rotation without disturbing the regular coupling of each pair of windings.

United States Patent [56] References Cited UNITED STATES PATENTS1,309,159 7/1919 Taylor 318/254 2,155,557 4/1939 Kranz 318/254 3,284,68711/1966 Schlenker.... 318/138 3,305,717 2/1967 Weiss 318/254 3,471,76210/1969 Urban 318/138 3,508,091 4/1970 Kavanaugh 310/49 PrimaryExaminer-G. R. Simmons Atrorney- Wenderoth, Lind and Ponack ABSTRACT: Anelectronically commutated motor having a multiple pole permanent magnetrotor. A position detecting means is located inside of the stator. Thisarrangement makes it easy to fabricate the stator and the positiondetecting means from the same sheet of laminated magnetic material withlittle material loss. A position detecting rotor having a plurality ofteeth with short-circuited conductors thereon modulates the couplingbetween the primary and secondary windings in the position detectingwindings during its rotation without disturbing the regular coupling ofeach pair of windings.

PATENTEU JUNZS l9?! SHEET 1 0F 3 INVENTORS KAZUTSUGU KOBAYASHI HISAYUKIMATSUMOTO YOSHIAKI IGARASHI' ATTORNEYS PATEN TED JUNZQISTI 3; 590.353

SHEET 2 [1F 3 FIG.2b

INVENTORS KAZUTSUGL) KOBAYASHI HISAYUKI MATSUMOTO YOSHIAKI IGARASHIATTORNEYS FIGBb PATENJTEU JUN29I9TI 3; 590.353

sum 3 [1F 3 FIG.3d l 2a 3 FlG.3h i3 h INVENTORS KAZUTSUGU KOBAYASHI H lSAYUKI MATSUMOTO YOSHIAKI IGARA SHI ATTORNEYS ELECTRONICALLY COMMUTATEDMOTOR HAVING AN OUTSIDE ROTOR AND AN INSIDE POSITION DETECTOR Thisinvention relates to improved electric motors particularly toelectronically commutated motors, and more particularly to those of theposition detecting type.

A motor which is compact, 'selfstarting, which has a preselecteddirection of rotation, which has a smooth torque,

and which is capable of operating on DC, is useful in electronicequipment for driving various movable components. Portable taperecorders, for example, require such a motor which in addition has verylittle torque ripple.

Up to this time many methods have been proposed relating to thecommutation of electric current flowing through the armature winding,which methods utilize electronic devices such as photosensitive elementscooperating with a light source and a rotating slit, magnetic sensitivedevices in combination with a permanent magnet, and an impedancecommutator utilizing a saturable reactor (US. Pat. No. 2,797,375) or themutual coupling of two coils (US. Pat. Nos. 1,971,188 and 2,644,916) alloperating on relatively high frequency alternating current.

Many deficiencies of mechanically commutated motors, such as relativelyshort life due to the wearing of brushes and the commutator material,generation of electric, electromagnetic, and sonic noise due to thesparking and chattering between the brush and the commutator, and energyloss due to the friction between the brushes and the commutatormaterial, are overcome by using any of the methods described above.

However, there do exist other difficulties in such brushless motors(hereinafter called electronically commutated motors). An electronicallycommutated motor employing photosensitive elements does not have a longlife or a high efficiency because the light source usuallyhas arelatively short life and poor efficiency of conversion of electricenergy to light energy. The most prominent feature of the optical systemwhich utilizes photosensitive elements is that it is easy to give thecommutation signal an on-off characteristic, or in other words, toprovide a discrete signal level for on-off operation.

A discontinuous commutating signal is necessary for-high efficiency. Themagnetic system and the impedance system are preferable to the opticalsystem from the stand point of their length of useful life, although thecommutation signal obtained by those systems is not a discontinuous one.

The impedance system, which is very inexpensive as compared to the othersystems, produces a commutation signal which has very poordiscontinuity, especially where the system has a simple construction.

ln addition to discontinuity, two important features of the commutationsignal are as follows:

i. No dead zone should exist when changing from one phase signal toanother phase signal. This will prevent incorrect starting.

ii. No overlap of one phase with another phase should exist.

This will prevent small torque ripple and promote high efficiency. Thesetwo conditions are very difficult to satisfy simultaneously with thefeature of good discontinuity in a commutation signal produced in anoptical system, and with the feature of poor discontinuity in acommutation signal produced in a magnetic system or an impedance system.

The electronically commutated motor, according to our copendingapplication, Ser. No. 803,218, has a commutation signal with the abovementioned important features without the disadvantages inherent in theuse of the optical system,the magnetic system or the impedance system.

Briefly described, the motor according to that invention utilizespolyphase rectification by a base-emitter circuit of a transistor. Thecollector current of the transistor is used directly as armature currentor is used as a commutating signal to control the power transistorswhich control the armature current.

The advantages of the invention of that copending application is toprevent overlap and dead zone, which are caused when using a graduallyvarying position signal. It is difficult to obtain a discontinuousposition signal but it is easy to obtain a gradually varying positionsignal.

It is an object of the present invention to provide an improvedelectronically commutated motor having an improved position detectingconstruction which is most suitable for a multiple pole motor. It is afurther object of the present invention to provide an electronicallycommutated motor in which, according to its construction, a sufficientlyprecise position signal can be obtained for a multiple pole motor. It isa still further object of the present invention to provide anelectronically commutated motor in which a stator core, a positiondetecting stator and a position detecting rotor can be punched from thesame sheet of silicon steel plate without loss of material.

Other objects and advantages of the present invention will, of course,become apparent and immediately suggest themselves to those skilled inthe art to which the invention is directed from a reading of thefollowing specification in connection with the accompanying drawing inwhich:

FlG. I is a schematic circuit diagram of an electronically commutatedmotor apparatus according to the present invention;

FIGS. 2a--2c are diagrams of a position detecting means according to thepresent invention;

FIGS. 3a3h are time diagrams for explaining the operation of the motorcircuits of FIG. 1.

In the embodiment shown in FIG. 1 a stator 1 has 24 slots and three setsof windings 2, 3 and 4 wound thereon and connected in a three phase starconnection.

A rotor 5 is rotatably positioned around the outside of the stator l andhas a plurality of poles, preferably more than four. A positiondetecting means is positioned within the stator l and comprises aposition detecting stator 6 positioned within the stator'l, a positiondetecting rotor 7 on the shaft of rotor 5, a set of primary windings(8a, 8b and 8c) and three secondary windings 9, 10 and 11 spaced aroundthe position detecting stator 6.

The electromagnetic couplings between the respective primary windings8a, 8b and 8c and the respective secondary windings 9, l0 and 11 arevaried by the position detecting rotor 7 which should have more than twoprojections and which is here shown as having four. Strongelectromagnetic coupling between these pairs of the primary andsecondarywindings is brought about four times during one rotation of the rotor 7,and each of the pairs consisting of a primary and a secondary windinghas one of the projections on the position detecting rotor 7 directlyfacing it when the rotor 7 is moved 30 from the position in which thepreceding projection faces the preceding pair of windings.

A transistor 20, resistors 21, 22 and 23, capacitors 24 and 25, and anoscillator coil 26 are connected in an oscillator circuit whichgenerates a comparatively high frequency Hz 100 KHz) AC signal.

The output signal of the oscillator circuit is fed to said primarywindings 8a, 8b and 8c through a capacitor 27.

Diodes 12, 13 and 14 are connected forwardly, with respect to currentflow from the secondary windings between the one ends of the respectivesecondary windings and the respective bases of transistors 28, 29 and30. Each of the other ends of the secondary windings 9, 10 and 11 areconnected to one another and connected to a point at which a resistor 18and a resistor 19 are connected to each other, said point hereinafterbeing designated as pedestal point 55. Resistors 18 and 19 are connectedin series across power supply lines from terminals 36 and 37.

Capacitors 15, 16 and 17 are connected between the bases of thetransistors 28, 29 and 30 and said pedestal point 55, respectively.

' The emitters of the transistors 31, 32 and 33 are each connected tothe other power supplying terminal 36.

The collectors of the transistors 31, 32 and 33 are connected to one endof said stator windings 2, 3 and 4, respec- .tively and each of theother ends of said stator windings 2, 3

and 4 is connected to the power supplying terminal 37.

Transistors 28, 29 and 30 have a polarity opposite to that oftransistors 31, 32 and 33; i.e., if transistors 31, 32 and 33 arePNPtype transistors, transistors 28, 29'and 30 are NPN-type transistorsand vice versa. Said oscillator circuit is energized by the current fedfrom the power supplying terminals 36 and 37.

In operation, the output signal of said oscillator circuit is fedthrough the capacitor 27 to the primary windings 8a, 8b and 80. A groupof voltages are induced in the secondary windings 9, l and l l insequence during rotation of the rotor 7.

Each of the voltages appearing at the ends of the respective secondarywindings has a frequency which is the same as the frequency of theoutput signal of the oscillator circuit, and has an amplitude varyingaccording to the rotational angle of position detecting rotor 7. It willbe understood that the position detecting rotor 7 modulates theamplitude of said voltage. The modulated signal is shown in FIGS. 3a3c.In FIGS. 3a, 3b and 3c, curves 45, 46 and 47 show the envelopes of theoutput signal of the respective secondary windings. These curves showthat said output signals are not greatly modulated, and

the envelopes form a three phase curve family.

Diodes l2, l3 and 14 rectify said output signals form the secondarywindings 10, 11 and 9 respectively, and capacitors l5, l6 and 17 filterout the carrier frequency, i.e. the frequency generated by theoscillator circuit. The voltages appearing between the pedestal point 55and the output side (cathode in this case) of the respective diodes, aredesignated e ,'e and e and are shown in FIG. 3d as curves 48, 49 and 50.The voltage appearing between the pedestal point 55 and the powersupplying terminal 37 is indicated as being e in FIG. 1.

The resistors 18 and 19 are given a resistance so that the currentflowing through them is comparatively large compared to the base currentof transistors 28, 29 and 30. The diodes l2, l3 and 14 have an outputimpedance which is low compared to the base circuit impedance of saidtransistors 28, 29 and 30. I

The output signal of the secondary windings 9, l0 and 11 are determinedby the value of the peak to valley voltage difference of the voltagese,, e, and e;,. When said peak to valley voltage difference is fromabout 0.5 volt to several volts, the transistors 28, 29 and '30 andresistor 34 act as a triple differential switching circuit.

The voltage e appearing between the common emitter circuit of saidtransistors 28, 29 and 30 and the power supplying terminal 37,corresponds to the greatest voltage of the combined voltage e +2 e '+e,,and e +e For example, the position of the position detecting rotor 7shown in FIG. 1 is such that e 41,, is the greatest of the threecombined voltages.

The transistor to which the highest base emitter voltage is applied, inthis instance transistor 30, feeds its emitter current to the resistor34 and the voltage e, is nearly equal to e -l-e 0.6 volts (when thetransistors 28, 29 and 30 are silicon transistors). This state isdesignated the ON state of the transistor 30. On the other hand, thebase emitter-voltages of the transistors 28 and 29 are very low and thebase current and the collector current can not flow. This state isdesignated the OFF state of transistors 28 and 29.

The collector current of transistor 30, which is in the ON state, issupplied to the base of transistor 33, which then is turned to the ONstate to supply collector current to the stator winding 4.

The current flowing through the stator winding4 generates a torque incooperation with permanent magnetized rotor 5. The rotation of the rotor5, and consequently the rotation of the position detecting rotor 7,varies the voltages e e and e,. If it is assumed that the torquegenerated by the current flowing through the stator windings 2, 3 and 4and the rotor 5 has a clockwise direction, then when the rotor 5 rotatesabout 15 from the position shown in FIG. '1, the output signals of thesecondary windings 9 and 10 have equal amplitudes and the voltages e;,and 2, become equal in value and the emitter current of transistor 30 isdecreased and the emitter current of transistor 28 is increased. I

At this point the two transistors 30 and 28 are in the same state, andboth of them feed their emitter current to the resistor 34. With furtherrotation of rotor 5, the emitter current of transistor 30 decreasesfurther and the emitter current of transistor 28 increases further. Thesum of the emitter currents of transistors 30 and 28 is determined bythe emitter voltage a and the resistance value of resistor 34. As thevoltage e follows to the base potential of transistors 30 and 28, itremains almost constant, as shown in FIG. 3e. When the emitter currentof each of transistors 30 and 28 has the same value, which is nearlyequal to one half of the emitter current of a single transistor when itis in the ON state, this state is called the transitional state oftransistors 30 and 28.

In the vicinity of the transitional state, the two transistors act as adifferential amplifier. But the maximum voltage difference of the twosecondary windings 9 and 10, i.e., the maximum difference of the inputsignal to the differential amplifying transistors 30 and 28, ispredetermined so as to be sufficiently large to overcome thedifferential operation and to drive one of the two transistors into theON state and the other into the OFF state. Thus the two transistorsoperate as a differential amplifier only for a very small rotationalangle of the rotor 7. Therefore, transistor 30 and 28 switch from an ONto an OFF state and vice versa almost instantaneously.

The change of the states of the transistors 28 and 29, and 29 and 30follow the same pattern as that for the transistors 30 and 28. The ONstate of each of the transistors 28, 29 and 30 continues for about 30 ofthe rotation of the rotor 5, respectively, and it is repeated four timesin sequence during one revolution of the rotor 7. Therefore the rotor 5generates torque in one direction all during its rotation. The collectorcurrents of the transistors 28, 29 and 30 are shown in FIGS. 3f-3h.Capacitor 35 eliminates undesirable parasitic oscillation.

The electronically commutated motor according to the present inventionas described above has many advantages over theconventionalelectronically commutated motor.

In the first place, because the rotor is rotatably positioned outside ofthe stator, it is possible to get higher inertia force than that of themotor in which the rotor is positioned inside of the stator andaccordingly fluctuation in rotating speed can be decreased. Further,because the outside rotor type motor can be smaller than the insiderotor type motor for generating the same torque, the electric loss inthe iron or copper of the motor of the. invention is less than that in aconventional motor and so higher efficiency is obtained.

Secondly, according to the invention, the detecting rotor is positionedinside of the stator, so that it is not necessary to provide added spacefor the position detecting rotor and the motor can be constructed so asto have a small size. On the contrary, the space inside of a stator isnot used in the conventional outside-rotor type motor. Further, byplacing the position detecting windings directly on the stator core,correlation between the positions of the stator windings and theposition detecting windings can be exactly adjusted so as to produce asuperior operation. Such a construction is suitable for mass productionat a low cost. In such an outside-rotor type motor, if the correlationof the positions of the rotor, stator winding position detecting rotorand the position detecting winding is not exact, it causes deviation inthe rotating speed and torque and reduces efficiency. However, accordingto the present invention, because the stator windings and the positiondetecting windings are placed on the same core as described above, thecorrelation of the positions of these windings can be made exact.Therefore, it is only necessary to adjust the correlation between therotor and the position detecting rotor and such adjustment can be easilycarried out because these rotors are mounted on the same shaft. Thearrangement of the position detecting means comprising primary andsecondary position detecting windings and a position detecting rotor isdetermined by the factors discussed below.

In FIG. 1, if one quarter rotation of the rotor 5 is defined as oneperiod, to rotate the rotor 5, it is necessary to provide electriccurrent to each of the three stator windings 2, 3 and 4 during oneperiod. For this purpose, it is usually necessary to arrange the primaryand secondary windings inside of one quarter portion of thecircumference of the position detecting means 6, and such arrangement isshown in FIG. 2a.

Referring to FIG. 2a, 55, 56 and 57 designate the combination of theprimary and secondary windings, respectively. As for the positioning ofthe primary and secondary windings of the primary and secondarywindings, positioning parallel and positioning perpendicular to theshaft are the two types possible. Even a small type motor can providespace for arrangement of the primary and secondary windings parallel tothe shaft. But the coupling flux from the primary windings to thesecondary windings has a component parallel to the shaft and soisotropic material such as ferrite is required. On the other hand, fromthe manufacturing standpoint, use of silicon steel laminations or plateis preferred because it can be easily stamped with high accuracy. Whenthe silicon steel plate is used, the parallel arrangement of the flux tothe shaft is inconvenient and the primary and secondary windings arerequired to be arranged perpendicularly to the shaft. When such aperpendicular arrangement is required, the arrangement as shown in FIG.2a is not suitable for a small type motor because the space factor islow and so the motor can not be miniaturized. FIG. 2(b) shows a novelmethod according to the present invention which overcomes this problem.Considering the periodicity of the position detecting rotor, it isunderstood that there are four points which become equivalent to each ofprimary and secondary windings 55, 56 and 57. These points aredesignated as A1, Bi and Ci (i=1, 2, 3 or 4), respectively in thefigure. Therefore, it is possible to arrange the windings at the platehaving extra space, for example, as shown (A,, C,, 8,). The symmetricalpoints (A,, B C have more extra space and they provide an effect owingto their symmetry. At first, undulation caused in the output ofthesecondary winding by the eccentricity of the position detecting rotor 7has a time lag of one-third ofa rotation with respect to the points A, Band C and consequently the period of the undulation becomes short andslightly random. Therefore, this undulation does not cause periodicallychanging current in the stator windings and so it does not become adeviation component of rotation of the rotor. The second effect is inthe manufacturing, that is, because of the construction beingsymmetrical it can be constructed effectively on a manufacturing line.

In manufacturing, each of the position detecting windings can beattached to any of these three points and further it is not necessary todistinguish the face and the back of the silicon steel plate. This isshown in FIG. 1. However, a position detecting rotor having such asimple construction does not always operate well, and in such anarrangement there can be caused a mutual interference between positionsA,, B and C For example, in the figure, the signals from the primarywindings 8a and 80 for the position signals ofA and B, induce theposition signals in the secondary winding 10 for the position signal ofC through the poles a and b, and d and h of the position detecting rotor7, respectively. Consequently, current flows to the stator windings atan irregular position and normal rotation is obstructed. To overcomesuch a problem, in the present invention, a short winding 58, which iscomprised ofa material having a high electric conductivity such ascopper plateor copper wire, is provided at the root of each pole of theposition detecting rotor 7 so as to isolate electromagnetically thesepoles, as shown in FIG. 2(0). The magnetic flux flowing to the root ofthe poles induces electric current in said short winding 58, and thatcurrent induces magnetic flux which negates the first-mentioned flux.Consequently, each of the poles is almost completely isolated and normalrotation can be carried out. Such a short winding can be provided easilyand cheaply without affecting the accuracy of the position detectingrotor. In the embodiment shown in FIG. 2(0), the root of each pole isdivided to three portions and provided with the short winding 58, but itis obvious that an undivided winding or a greater number of dividedwindings having a similar effect.

As described hereinbefore, the electronically commutated motor accordingto the present invention can be manufactured with high accuracy and highefficiency. Moreover, the motor of the invention can provide a sureoperation with very constant torque, in which the error within themanufacturing accuracy hardly causes deviation during one rotation.

Although a preferred embodiment of the invention has been set forth indetail, it is desired to emphasize that it is not intended to beexhaustive or necessarily limitative; on the contrary, the disclosureherein is for the purpose of illustrating the invention and thus toenable others skilled in art to adapt the invention in such ways as meetthe requirement of particular applications, it being understood thatvarious modifications may be made without departing from the scope ofthe invention.

What we claim is:

1. An electronically commutated motor comprising a rotor having amultiple pole permanent magnet;

a stator having a set of stator windings and being positioned within theinner periphery of said rotor, said rotor being rotatable to rotatearound the outside of said stator;

a position detecting means having a position detecting stator and aposition detecting rotor, said position detecting stator beingpositioned within the internal periphery of said stator and saidposition detecting rotor having a plurality of teeth and being rotatableinside of the position detecting stator and with said rotor, saidposition detecting means further having signal producing means producinga set of position signals under the influence of said teeth forindicating the relative rotational position of said stator and saidrotor; and

a current control means coupled to said signal producing means which iscontrolled by said position signals from said position detecting meansand coupled to said stator windings for controlling the current flowingthrough said stator windings.

2. An electronically commutated motor as claimed in claim 1 in whichsaid position detecting stator includes plurality of sets of primary andsecondary windings and said position detecting rotor modulates thecoupling between said primary and secondary windings.

3. An electronically commutated motor as claimed in claim 2 in whichsaid position detecting rotor has short circuited conductors on eachtooth interlinking the path of magnetic flux from one tooth to anothertooth.

4. An electronically commutated motor in accordance with claim I whereinsaid position detecting rotor is made of laminated magnetic material.

5. An electronically communicated motor in accordance with claim Iwherein said permanent magnet has 2n poles, said stator windings arearranged in m phases, said position detecting rotor has n teeth, andsaid signal producing means comprises m pairs of position detectingwindings positioned every 360/m/maround the inside of said positiondetecting stator.

1. An electronically commutated motor comprising a rotor having amultiple pole permanent magnet; a stator having a set of stator windingsand being positioned within the inner periphery of said rotor, saidrotor being rotatable to rotate around the outside of said stator; aposition detecting means having a position detecting stator and aposition detecting rotor, said position detecting stator beingpositioned within the internal periphery of said stator and saidposition detecting rotor having a plurality of teeth and being rotatableinside of the position detecting stator and with said rotor, saidposition detecting means further having signal producing means producinga set of position signals under the influence of said teeth forindicating the relative rotational position of said stator and saidrotor; and a current control means coupled to said signal producingmeans which is controlled by said position signals from said positiondetecting means and coupled to said stator windings for controlling thecurrent flowing through said stator windings.
 2. An electronicallycommutated motor as claimed in claim 1 in which said position detectingstator includes plurality of sets of primary and secondary windings andsaid position detecting rotor modulates the coupling between saidprimary and secondary windings.
 3. An electronically commutated motor asclaimed in claim 2 in which said position detecting rotor has shortcircuited conductors on each tooth interlinking the path of magneticflux from one tooth to another tooth.
 4. An electronically commutatedmotor in accordance with claim 1 wherein said position detecting rotoris made of laminated magnetic material.
 5. An electronicallycommunicated motor in accordance with claim 1 wherein said permanentmagnet has 2n poles, said stator windings are arranged in m phases, saidposition detecting rotor has n teeth, and said signal producing meanscomprises m pairs of position detecting windings positioned every360/m*/m* around the inside of said position detecting stator.